1. Field of the Invention
The field of the present invention is holographic displays, specifically holographic displays with multiple images, including methods and apparatus for achieving such displays.
2. Description of the Prior Art
Holographic displays, including holographic displays using a stereographically recorded holographic images, are known to be useful for applications where the design and evaluation of complex structures such as homes, offices and automobiles require the presentation or evaluation of three dimensional designs See, for example, U.S. Pat. Nos. 3,832,027 to King and 4,834,476 to Benton. Such holographic images can be synthesized from a plurality of computer-generated two-dimensional views and then stereographically recorded on a recording medium. The holographic images in the developed recording medium can then be projected into a display area for evaluation by a viewer who, at least to some extent, is able to view different portions of the three-dimensional image by moving in relation to the display area. In this fashion, the viewer is able to see different aspects of the image in proper perspective leading to a better appreciation of the spatial relationships of parts of the image.
Recently, holographic displays have been used for man-machine interfaces in order to present, for example, the operational status of various parts of a copier machine. See U.S. Pat. No. 4,896,929 (Haas et al.). The problem in using holographic displays in traditional man-machine interfaces is to display all of the information of interest while not having the holographic image cluttered with too much information. Liquid crystal or LED pointers can highlight certain parts of an image though cluttering an image may still reduce the effectiveness of the display. While color and reference beam angle multiplexing might be used to show separately several images, each of which contains different information, thus reducing the density of information in any single image, a reduced diffraction efficiency and intermodulation impose a practical limit on the number of separate multiplexed images to four or less for typical copier-printer displays. Because such applications traditionally require at least 30 separate pieces of information to be displayed, the problem of a cluttered display still exists.
Simply blocking or masking part of a holographic image created in the conventional manner such as with a twisted nematic liquid crystal display is not very effective in simplifying the display for at least two reasons. First, the blocked areas are dark, distracting from the rest of the display. Second, and of particular significance, the liquid crystal display or other masking element exists only in one plane. Therefore, as a viewer changes viewing position with respect to the display it is possible to see objects lying behind the blocked plane, the so-called "look around" effect. This effect is not aesthetically pleasing and ultimately leads to confusion as to the meaning of the blocked portions of the display.